1. Field of the Invention
This invention, which is a result of a contract with the U.S. Department of Energy, relates generally to processes for the removal of gaseous fluorine from gas streams. More particularly, it relates to processes for the direct recovery of fluorine from gas streams which also contain, for example, nitrogen and/or oxygen.
As used herein, the phrase "direct recovery of fluorine" refers to dynamic processes in which a stream of fluorine-containing gas is passed through a bed of material to effect removal, or absorption, of the fluorine by chemical reaction with the bed, the absorbed fluorine subsequently being recovered as a gas by heating of the bed.
2. Prior Art
Various industrial processes utilize elemental fluorine (F.sub.2) and, as a result, generate vent streams containing excess or unreacted F.sub.2. One such process is the manufacture of uranium hexafluoride for use as the process gas in gaseous diffusion plants for the separation of uranium isotopes. In that process, UO.sub.3 or U.sub.3 O.sub.8 is reacted with excess F.sub.2 at an elevated temperature to produce UF.sub.6. The vent stream for the process typically comprises from 50 to 75 mole percent F.sub.2, the remainder of the gas consisting essentially of oxygen. The fluorine emissions from such processes must be controlled for environmental reasons. Furthermore, the value of the discharged F.sub.2 is a significant operating cost. In general, the approach to decreasing F.sub.2 emissions has been to convert them chemically to less toxic forms, such as CaF.sub.2 or CF.sub.4. However, such processes do not permit the re-use of the captured F.sub.2 and in some instances generate significant quantities of solid waste products.
As compared with chemical-conversion processes of the kind referred to above, processes for the direct recovery of fluorine from vent streams provide at least two important advantages: they minimize the generation of solid waste and they permit re-use of the captured fluroine. A previously known process for the direct recovery of fluorine from vent streams is based on the following reversible reaction: ##EQU1## That is, the F.sub.2 -containing gas is passed through a fixed bed of cobalt difluoride at an elevated temperature to convert the latter to the trifluoride, thus absorbing fluorine; subsequently, the bed is heated to a higher temperature to evolve the absorbed fluorine. Unfortunately, that process is not entirely satisfactory because of excessive corrosion of materials of construction, owing to the high temperature of regeneration.
U.S. Pat. No. 3,989,808 (issued on Nov. 2, 1976) describes a static method for the preparation of pure fluorine gas. In that method a 3:1 mole-ratio powder mixture of KF to NiF.sub.2 is placed in a reactor and contacted with impure gaseous F.sub.2 at superatmospheric pressure and 500.degree. C. to absorb the F.sub.2 and form a nickel-fluoride complex. The reactor temperature then is lowered to 250.degree.-300.degree. C., and additional impure F.sub.2 is admitted to maintain a pressure of several atmospheres therein, with the result that the complex absorbs the F.sub.2 in a matter of hours. The reactor temperature then is increased to about 400.degree. C. to decompose the complex and evolve pure F.sub.2 at a pressure of about 20 atmospheres. The method is based on the following reversible reaction: ##EQU2##